Growth of North Sea cod, Gadus morhua

Abstract

As part of a study of consumption and production of North Sea cod an analysis is presented of the growth of North Sea cod, utilising market sampling, research vessel survey, and tagging data. Bertalanffy growth curves were estimated from market samples for the southern and northern North Sea separately. The growth rate appears to be lower and the t 0 value higher in the northern North Sea, but ultimately a higher length is reached than in the southern North Sea. This coincides with apparently better feeding conditions for large cod in the northern North Sea. Analysis of seasonal changes of the rate of growth suggested a higher growth rate in winter in the southern North Sea, but no consistent pattern was observed in the northern North Sea. Apparently growth in cod is not restricted to a particular time of the year. Mean lengths of juvenile cod caught during research vessel surveys were compared with the growth curves estimated from commercially sized cod. From one year old onwards there was good agreement, but during the first year of life the curve underestimated real growth. In juvenile cod growth could not be shown to be density dependent. Analysis of growth in tagged cod showed that age reading from North Sea cod otoliths presents a reliable tool for growth studies. Information on length-weight relations showed that there is no evidence to reject isometric growth in cod. Problems in interpretation of length-weight data are discussed. The parameters of the Bertalanffy growth curve are estimated at L ∞ = 110.8 cm; W ∞ = 14150 g; K = 0.333; t 0 = 0.77 year for the southern North Sea and L ∞ = 118.7 cm; W ∞ = 17390 g; K = 0.269; t 0 = 0.87 year for the northern North Sea. A mean condition coefficient of k = 0.0104 applies to both North Sea and northwestern Atlantic cod.